1. Field of the Invention
The invention concerns a d-c control circuit of a type comprising: a controllable main valve which is arranged in one of the two connecting lines between a d-c voltage source and a load; a quenching branch for the main valve which includes a commutation capacitor, a commutation choke and a controllable commutation valve in series and which is connected parallel to the main valve in the blocking direction of the commutation valve; a flyback (or swing-back) valve connected antiparallel to the main valve; and a bypass valve which is arranged parallel to the load.
2. Description of the Prior Art
Such a d-c control circuit is known, for instance, from the German Pat. No. 1,242,289. It is used particularly for operating a d-c machine. In particular, when used in this manner the d-c control circuit only permits load current to flow in one of the two current directions. Therefore, operation is possible only in one of the four quadrants of a coordinate system which is formed by the voltage and the current of the load. In many applications, however, such as, for instance, for a propulsion drive with a d-c machine, current reversal and operation with the other current direction is desired. In particular, such other current direction is needed to provide the required braking operation to the propulsion drive system.
It is therefore an object of the present invention to provide a d-c control circuit which permits and can operate during a reversal of the current direction at the load. In other words, the d-c control circuit of the present invention is to be designed to provide two-quadrant operation.